1. Field of the Invention
The present invention relates to a removable ink composition used for prints on plastic containers and plastic films, and more particularly, to an ink composition which is used for prints on plastic films and plastic containers and can be removed and decolorized easily by treating the ink composition with a dilute aqueous alkali solution and a process for removing the ink composition from articles printed with the ink composition.
2. Description of the Related Arts
Various containers made of polyethylene terephthalate have been used widely as containers of refreshing drinks, cosmetics, seasonings and drugs. Trade names and descriptions about contents are directly printed on the surface of these containers or printed on labels which are attached to the surface of these containers.
Environmental pollution caused by disposed containers, labels and packaging bags made of plastics which are hardly decomposed in the natural condition has recently become a problem and it is urgently required to decrease the amount of such disposed articles. Recovery of plastic materials from containers of refreshing drinks which are generally called PET bottles has been advanced considerably. The recovered plastic materials are fabricated again to prepare secondary products.
Plastic articles such as containers from which the materials are recovered are attached with printed shrink films. Secondary products fabricated using the recovered plastic materials are not free from various colors unless inks used in the prints are removed or decolorized before the fabrication. Therefore, it is necessary that inks used in the prints be removed or decolorized to obtain secondary products having properties similar to products which are made of new materials.
As the process for decolorizing printed plastic articles, for example, a process in which plastic articles are treated with a cleaning liquid containing an organic solvent which dissolves or swells the coating material of the prints and a caustic alkali as essential components, is known (Japanese Patent Publication Showa 52(1977)-26549).
This process has a drawback in that the organic solvent contained in the waste liquid must be separated and recovered and the operations of separation and recovery must be additionally conducted. Moreover, gravure ink compositions generally used for plastic films and plastic containers are prepared by adding pigments to a vehicle containing a urethane resin or an acrylic resin as the main component, cellulose resins, vinyl acetate resins and rosin. Therefore, it is difficult to remove or decolorize the ink compositions used in the prints by treating the ink compositions with an aqueous solution of sodium hydroxide having a concentration of 1 to 3%. It is urgently required that the above problems be solved and colorless materials be recovered from plastic films and plastic containers by treating ink compositions used in the prints on plastic films and plastic containers with a dilute alkali solution.